German firm BioNTech warned Tuesday there is no data backing the "safety and efficacy" of delaying the second shot of its Covid-19 vaccine beyond three weeks, as some countries push back the jab to give more people their first dose.

BioNTech, which developed the vaccine with US giant Pfizer, said its clinical data showing 95 percent efficacy was based on a two-dose schedule separated by 21 days.

-The safety and efficacy of the vaccine has not been evaluated on different dosing schedules- it said.

-Although data... demonstrated that there is a partial protection from the vaccine as early as 12 days after the first dose, there is no data to demonstrate that protection after the first dose is sustained after 21 days.-

Faced with limited supplies of the vaccines, Denmark said Monday it would space out the jabs by up to six weeks, while Britain has said it would wait up to 12 weeks before giving the second jab.

Germany too is mulling pushing back the second shot to beyond 21 days.

Proponents say that practice would allow more of the population to get their first jab.

-While decisions on alternative dosing regimens reside with health authorities, we believe a second dose is required to provide the maximum protection against the disease- BioNTech said.

Mainz-based BioNTech said it remains committed to continuing "dialogue with regulators, health authorities and governments" on "any public health decisions".